Field of the Invention
The present invention relates to a position detection apparatus that detects a position of an object.
Description of the Related Art
Previously, an encoder is used as a position detection apparatus provided in a machine tool, an FA equipment, or the like. In an incremental encoder, a reference point is detected by using a reference position mark provided at a specific position on a scale, and a position within a stroke is detected based on a moving amount relative to the reference point.
Japanese Translation of PCT International Application Publication No. 2012-500981 discloses a method of detecting a reference point on a scale. In the method disclosed in Japanese Translation of PCT International Application Publication No. 2012-500981, a solitary wave (i.e., signal having an amplitude only at an origin) which is generated at a reference position on the scale is used, and a reference position pulse is generated based on a result obtained by comparing this solitary wave with a threshold value. Furthermore, reversed-phase pulses where positive and negative signs are reversed to each other are generated to perform a differential calculation, and thus a pulse position of a reference point signal does not easily vary even when variation of an amount of light emitted from the light source.
However, in a conventional typical reference point detection method, a fine light receiving element array is needed to obtain a highly-accurate reference position signal, and accordingly manufacturing a sensor requires cost. To solve this problem, a configuration where a light receiving element with a large area is disposed behind a fine optical grating is known. In this configuration, however, it is necessary to use different grating areas to generate the reversed-phase pulses where the positive and negative signs are reversed to each other, and thus an apparatus increases in size.